It has heretofore been proposed to test for air leaks in closed and semi-closed structures ultrasonically in a soundproof enclosure. U.S. Pat. No. 4,809,538 and application Ser. No. 222,018 filed July 21, 1988, and now U.S. Pat. No. 4,852,390; both assigned to the assignee hereof, disclose leak detection apparatus in which a transfer line sequentially feeds a plurality of closed or semiclosed workpieces, such as vehicles wheels, oil pans, valve covers or catalytic converter housings, to a test station. An elevator at the test station lifts each workpiece in sequence from the transfer line to an elevated position at which the workpiece is disposed within and enclosed by a sound chamber. The sound chamber is formed by a cup-shaped enclosure wall having an open bottom edge that sealingly engages the elevator in the upper or test position of the latter. An air supply selectively feeds air under pressure into the workpiece enclosed in the chamber, and one or more microphones ultrasonically detect air leakage from within the workpiece.
Automated test apparatus of the described character has enjoyed substantial commercial acceptance and success. However, further improvements remain desirable. For example, problems have been encountered in attempting to calibrate the leak detection equipment to a maximum allowable flow rate at a given air pressure specified by the workpiece manufacture or customer. Workpieces that are known to exceed allowable leakage flow rate have been employed for calibration proposes. However, while such a "know leaker" can be employed for qualitative examination of the microphone outputs, they cannot be readily employed for qualitative control proposes to set a microphone output threshold corresponding to the desired maximum allowable flow rate for an acceptable workpiece. Further, the air passages in the test piece become clogged with dirt and debris, whereupon the test piece must be replaced by another known leaker that may have completely different leakage characteristics.
Accordingly, it is general object of the present invention to provide a method and apparatus for calibrating leak detection equipment of the described character that provide an accurate and repeatable qualitative vehicle for calibrating the equipment to a pre-specified maximum allowable flow rate for satisfactory workpieces, that may be readily employed periodically during an automated test operation for recalibrating the equipment, that may be readily modified in the event of a change in specified allowable flow rate, and that may be readily cleaned or repaired in the event of clogging by dirt or other debris in the test equipment.